1. Field of the Invention
The present invention is directed to a computed tomography apparatus of the type employing a surface detector for detecting radiation attenuated by an examination subject during the course of a spiral scan.
2. Description of the Prior Art
In computed tomography systems currently used in medical technology, the projection data required for a sectional image are exposed by passing the entire beam "bundle" from the X-ray source through one or more diaphragms so that only a thin fan beam remains, and detecting the radiation from this fan beam which is transmitted through the subject using a single detector row. The length of the individual detector elements in the z-direction (direction of the system axis) is dimensioned so that the detector elements can fully receive the radiation for the largest slice thickness that can be set (a 10 mm slice is standard).
Different slice thicknesses are produced by means of corresponding settings of the diaphragm near the tube and a diaphragm at the detector side. With an arrangement of this sort only the data for one slice can be recorded, and thus the X-ray radiation emitted by the X-ray source is used very inefficiently. For three-dimensional exposure techniques, the volume that can be acquired is generally limited by the available continuous power of the X-ray source. The required exposure times, or examination times thus are correspondingly long.
Limitations of this sort in the measurement system are largely overcome if, according to U.S. Pat. No. 5,291,402, a surface detector is used. A surface detector of this sort is a two-dimensional array of detector elements (mosaic), i.e. it is formed from several parallel detector rows, so that, in place of a thin fan beam, an X-ray beam bundle that is also extended in the z-direction can be used for imaging. In contrast to a conventional single-row detector, a surface detector also contains detector elements that are spatially separated in the z-direction. Given a rotation of the measurement system, this allows many slices to be exposed simultaneously, depending on the extension of the surface detector in the z-direction. Adjacent rows of the surface detector thereby acquire adjacent slices. The length of the detector elements in the z-direction for this type of detector thus is chosen so that a detector row acquires the smallest desired slice.